Purification of magnesium chloride solutions

ABSTRACT

A BITTERN WHICH IS SUBSTANTIALLY FREE OF SODIUM AND POTASSIUM VALUES IS READILY OBTAINED BY ADJUSTING THE MGCL2 CONCENTRATION OF THE BITTERN TO APPROXIMATELY 30% BY WEIGHT, COOLING THE ADJUSTED BITTERN TO ABOUT -10*C. TO 20*C., PREFERABLY 0*C., MAINTAINING THE BITTERN AT SUCH TEMPERATURE UNTIL CRYSTALLIZATION IS SUBSTANTIALLY COMPLETE, AND THEN RECOVERING PURIFIED MAGNESIUM CHLORIDE BITTERN.

United States Patent 3,676,077 PURIFICATION OF MAGNESIUM CHLORIDE SOLUTIONS Norman Bell and Hung-Kei H. Lam, Walnut Creek, Calif.,

assignors to Kaiser Aluminum & Chemical Corporation, Oakland, Calif. No Drawing. Filed Aug. 28, 1969, Ser. No. 853,971

Int. Cl. C01f 5/30 US. Cl. 23296 4 Claims ABSTRACT OF THE DISCLOSURE A bittern which is substantially free of sodium and potassium values is readily obtained by adjusting the MgCl concentration of the bittern to approximately 30% by weight, cooling the adjusted bittern to about C. to 20 0., preferably 0 C., maintaining the bittern at such temperature until crystallization is substantially complete, and then recovering purified magnesium chloride bittern.

BACKGROUND OF THE INVENTION In producing magnesium metal by electrolysis of a fused magnesium salt in the so-called light bath process wherein the cell charge is of lower density than the magnesium metal produced, it is very important that the content of heavier salts, especially sodium and potassium salts, be kept very low, suitably not over about 0.5% by weight of both salts, based on total weight of the feed. These salts occur as normal impurities in high magnesium chloride brines available from salt-producing and potash-making works and the content thereof is generally higher than is tolerable in the light bath process. There is a need for an inexpensive and simple way of lowering the sodium and potassium content of the brines to enable them to be converted into suitable light bath magnesium metal electrolysis cell feedstock.

In the operation of such process, these high density salts tend to accumulate in the cell. This can increase the density of the molten salt bath to the point where the metal as it is produced goes to the top of the electrolysis zone. For proper operation, the cells are designed to withdraw metal product from the bottom of the electrolysis zone, and furthermore, the metal is protected in that location by the overlying salt layer and there is less occlusion of salt in the metal layer. Thus, the preservation of the density relationships is of great importance, to maintain the metal as the bottom layer. In addition, since low density lithium salt also tends to accumulate in the cell, it is necessary to maintain a satisfactorily high ratio of lithium to sodium plus potassium content in the cell feed to provide and to maintain the needed density differential between the bath and the metal. This can be achieved with brines having substantial lithium contents by reducing the amount of sodium and potassium in the brine.

The above objects and others are attained by the practice of the present invention, as will become apparent from the description below.

SUMMARY OF THE INVENTION This invention concerns a method of preparing a magnesium chloride solution for making a feed for an electrolytic cell wherein magnesium metal is produced by electrolysis of a fused magnesium salt; and, more particularly, it relates to a method for producing such magnesium chloride solution of satisfactory low level of sodium and p0- tassium content, especially for use in a light bath electrolytic cell.

"ice

In the light bath process, the electrolyte contains essentially a major portion of lithium chloride which is of lower specific gravity than magnesium, and a minor but effective portion of the heavier magnesium chloride which is the source of the metal. In addition, a very small amount, preferably about 1% by weight of the bath of an alkali or alkaline earth metal fluoride is often included to assist in metal collection. When the naturally included sodium and potassium impurities increase or build up in the cell charge or in the cell bath, there must be added more lithium chloride to maintain the proper density relationships so that the metal collects as a lower layer in the cell. The density relationships of the individual compounds or substances present in such cell are shown in Table 1.

TABLE I Density of Magnesium and Various Cell Salts at 750 C. and 800 0.

Density, LiCl Density, LiCl Substance g./cc. equiv. g./cc. equiv.

Mg metal 1. 5632 1. 5500 1. 6671 l. 77 1. 6520 l. 63 1. 5394 0. 342 1.5102 0. 1. 5838 0. 895 l. 5567 0. 726 1. 4414 l. 4198 2. 7790 9. l5 2. 7595 8. 07

Ganz, G. 7., et a1. Molten Salt Data-Electrical Conductance, Density andViscosity. Troy, N.Y.R.P.I. (July, 1964) (U.S.A.F.O.S.R. 64-00394 U.S.D.D.C.A.D. 605, 938).

b Weight of LiCl needed to bring .454 kg. of the substance to a density 0.050 g./cc. less than that of the metal to provide the needed settling driving force.

Thus, from Table I it can be seen that, in a light bath cell, the presence of appreciable amounts of the potassium and sodium salts increases the requirements of LiCl significantly. It has been recognized that a difi'erence of at least about 0.034, and advantageously of about 0.050, g./cc. between the density of the metal and that of the electrolyte bath eflFects good separation of the metal as the bottom layer. This difference is most economically obtained in the absence of appreciable amounts of sodium and potassiurn.

According to the present invention an electrolyte and density differential under cell operating conditions are provided by substantial reduction of the sodium and potassium contents by a simple and efficient low temperature equilibration of the starting magnesium chloride brine. By this invention, a brine containing chlorides of magnesium, lithium, sodium and potassium salts are adjusted to a MgCl content of approximately 30% by weight. This can be accomplished by the standard procedures. The bittern or concentrated brine can be prepared by adding MgCl to a more dilute magnesium chloride solution or brine to bring the content thereof to the desired level; or by solar evaporation of the dilute brine in solar ponds, or more expeditiously in a plant by heating the brine, for example, from about 99 C. to about 116 C. This latter point is indicated in a practical manner by the presence of a detectable amount of bischofite, MgCl -6H O or by determining the boiling point of the solution. A small sample can be quickly tested, and brines of this composition will boil at -126 C., at a MgCl concentration of about 30% by weight. If the boiling point of the specimen is too high, water is added to lower the salt concentration. If the boiling point is too low, MgCl is added to raise the concentration.

The bittern is then cooled to a temperature of from about 10 C. to about 20 C. to precipitate out sodium and potassium chlorides, and the supernatant solution is separated off to recover a purified MgCl bittern. When the bittern is cooled to C., the preferential precipitation of sodium and potassium chlorides is very eifective. The precipitated salts can be recovered for further processing or can be discarded. Separation can be effected by any desired method, such as, for instance, screening, decantation, centrifuging or other.

In the preparation of light bath cell feedstock, the lithium content of some starting brines is low and in this event lithium chloride is added as desired to provide the proper concentration thereof in the bittern and at least the proper ratio of lithium to the amounts of sodium and potassium which accumulate in the electrolysis cell during operation thereof. A typical light bath electrolyte, anhydrous, contains from to 38% MgCl less than one percent of an alkali or alkaline earth metal fluoride and the balance lithium chloride. Some natural brines contain the desired amount of lithium but where lithium chloride must be added, it can be recovered from the bath dippings which are periodically or continuously removed from the operating electrolysis cell, e.g. in order to ofiset the accumulation therein of sodium, potassium or both.

The bittern containing at least 30% MgCl is fed to a cooling zone and cooled to a temperature of from C. to 20 0., preferably to about 0 C. In the event that concentration is effected by heating to slightly below the boiling point as described above, it is desirable to cool the heated bittern to about from 21 C. to 27 C. and then to adjust its MgCl concentration to at least 30%, advantageously 32% to 32.5%, by wt., prior to the cold equilibration at 1 0 C. to 20 C., as described herein. In an operation which is preferred because of the ease and completeness of crystallization and removal of the undesired amounts of sodium and potassium salts, the bittern is fed continuously to a cooling crystallizer where it is simultaneously cooled and agitated to elfect good deposition and separation of the precipitated crystals.

The supernatant solution is separated off from the precipitated crystals to recover a purified magnesium chlo ride bittern which contains less than 0.5% of both sodium and potassium, expressed as chlorides. Such bittern is useful for conversion to anhydrous condition for use as feedstock for an electrolytic cell wherein magnesium metal is made by electrolysis of fused magnesium chloride and particularly where such stock is for a light bath operation and contains a major portion of lithium chloride.

It is an advantage of the process of the present invention that the undesired amounts of sodium and potassium can be reduced or substantially eliminated in high magnesium chloride brines by a simple process which is adaptable to a variety of plant conditions. The process can be carried out in pond systems, for instance, in a region where solar heat in the warm or hot season acts to concentrate the brine by evaporation, and in the cold season the pond temperature is sufiiciently reduced to effect the low temperature selective precipitation of the sodium and potassium values. In such pond system, the purified bittern can be transferred to holding ponds, where no further precipitation occurs upon increase of the ambient temperature, and the bittern can be protected from winds and the atmosphere by suitable layers of protective, immiscible liquid such as a one-half to three-inch layer of a water-immiscible oil of a specific gravity of from 1.1 to 1.2. Such layer protects from contamination and also prevents further evaporation of the bittern. The oil layer can be of coal tar, residual oil and various high boiling chlorinated organic solvents.

DETAILED DESCRIPTION OF THE INVENTION The following specific description is given for purposes of illustration of some modes of carrying out the present invention.

A brine obtained from the Bonneville Salt Flats in Utah, which has been processed in the usual way to precipitate out sodium chloride, has the following composition:

The brine is heated at about 40 C. until a test sample shows a trace of bischofite, MgCl -6H O. The MgCl content of the bittern so produced is 32.2% and analysis shows the following: KC10.07%, NaCl 0.24%, LiCl 1%, MgSO, 2.2%.

The bittern is cooled to about 70 F. and decanted off from any solids which deposit at this point. Sufficient solid MgCl is added to bring the content thereof in the bittern to 32%. A portion of the clear supernatant liquor is then sent to a jacketed crystallizing tank fitted with a stirrer where it is cooled to 5 C. and held at this temperature with constant stirring for 200 minutes, or until precipitation has substantially ceased. The clear solution is decanted off and recovered. Three other portions of the bittern are treated in exactly the same manner except that they are cooled to, respectively, 0 0., 5 C. and 15 C. The contents of the various salts in the recovered clear bitterns are shown in Table II.

TABLE II Composition of Saturated Magnesium Chloride Brines alter Low Temperature Equilibration The clear solutions are then dried and made up into typical cell electrolytes for a light bath operation, the electrolytes having the compositions shown in Table III.

TABLE III The Efiect 0i Equilibration Temperature on the Composition and Densities of the Resulting Electrolytic Baths Equilibration temperature 5 0. 5 C. 15 0.

Concentration inweight percent:

h g z 12. 0 12. 0 12. 0 2. 0 2. 0 2. 0 65. 0 63. 2 58. 6 17. 5 18. 1 18. 3 3.5 4. 7 9.1

Bath density at 750 0 1.5236 1.5256 1.5302 Density difierential between bath and metal at 750 C 0. 0396 0. 0376 0. 0330 Bath density at 800 C 1. 5016 1.5035 1.5077 Density difierential between bath and metal at 800 C 0. 0484 0. 0465 0. 0423 In this specification and claims all parts and percentages are by weight unless otherwise indicated. The removal of sodium and potassium values in the present process to susbtantial freedom therefrom for the purposes of the electrolytic bath is enhanced by the presence of the substantial amounts of lithium salts in the brine treated. It will be understood that the above specific description has been given for purposes of illustration only and that variations and modifications can be made therein without departing from the scope of the appended claims.

Having now described the invention, what is claimed is:

1. A method for removing sodium chloride and potassium chloride from a solution of magnesium chloride which contains chlorides of sodium, potassium and lithium comprising:

(a) concentrating by evaporating the solution to a magnesium chloride concentration of at least about 30% by weight, by heating the solution to a temperature References Cited from tlbOllt 99 C. to about 115 0-, (b) cooling the solution to a temperature from about to about 2 C. 3,096,152 7/1963 Hadzeriga 23-91 (c) readjusting the magnesium chloride concentration 5 1,215,546 2/1917 f g of said solution to at least about 30% by weight, 1,593,038 7/1926 i ee (d) continuously feeding the solution through a cool- 1,863,751 6/1932 Kipper 23 9 ing zone wherein the solution temperature is lowered 1,878,586 9/1932 Klpper 8 to approximately 0 O, OTHER REFERENCES (e) holding the solution at a temperature of approxi- 10 mately 0 C. for at least about 180 minutes, and Journal of Apphed chem" 316 193] (f) separating solid matter from solution. N RMAN YUDKOFF P Examine. 2. A method of claim 1 wherein the solution is evapo- O y rated to provide a magnesium chloride concentration of at SILVERBERG, Assistant am r least 32% by weight. 15

3. A method of claim 2 wherein the readajusted magnesium chloride concentration is at least 32% by Weight. 304

4. A method of claim 2 wherein the solution of magnesium chloride contains magnesium sulfate. 

